OK thanks, I think you've answered my question between this message and the links you provided. On Tue, Jul 5, 2011 at 18:29, Gregory Farnum <gregory.farnum@xxxxxxxxxxxxx> wrote: > On Mon, Jul 4, 2011 at 10:17 AM, Vasilakakos Giorgos <sugoruyo@xxxxxxxxx> wrote: >> Hello folks, I have a question: >> I know files get mapped to objects and those objects to OSDs but how >> exactly do we go from one to the other? >> What I mean is, given the filename I want to access how does Ceph find >> the relevant objects? What exactly does CRUSH do? > The mapping between files and objects is by inode number and doesn't > involve the filename at all (if it did you'd have to move objects just > because you'd renamed a file!). >> Supposedly, the client first asks the MDS for an inode number, what then? >> How are the object and PG IDs constructed and where exactly does CRUSH come in? >> My impression is that the OID is made from the name, inode and >> probably some kind of information about the striping of the file, >> what about the mapping of an object to a PG? >> Are PGs mapped to OSDs by means of CRUSH, or are objects mapped to PGs >> by CRUSH? How are PGs assigned to OSDs then? > The object name is just the inode number postfixed by a sequence > counter for each object included in the file. (If it included the file > name, you'd have to rename objects and move them just because you > renamed the file. Not good.) The information about striping is > contained in object metadata that the OSDs keep. > I believe that by default the striping is just linear, so if your > object is less than 4MB it will take one object; if it is more than > 4MB it will take (ceiling(size/4MB)) objects named 2000000001.0, > 2000000001.1, 2000000001.2.... > You could set up other striping strategies though, so that it striped > eg 1MB over each object and did 4 stripes per object, but the objects > would still be named the same. > CRUSH then maps the OID onto a PG and maps the PG onto an OSD. (So, > objects are mapped by CRUSH into PGs, and PGs are mapped by CRUSH onto > OSDs.) > -Greg > -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
